FIG. 4 is a block diagram showing an example of the last stage of a driver circuit in the conventional technology having a complementary configuration. In this example, the last stage of the driver circuit is comprised of a bias circuit 40, output elements 31 and 32, and an impedance matching resistor 4. FIG. 5 shows a more detailed structure of the driver circuit of FIG. 4 which is used, for example, as a pin driver circuit in a test channel of a semiconductor test system. In such a semiconductor test system, the pin driver circuit is to apply a test signal to a corresponding one of device pins of a semiconductor device under test (DUT). The example of driver circuit in FIG. 5 has no temperature correction circuit.
The output elements 31 and 32 in this example are formed with CMOS transfer gates. The output elements 31 and 32 in the last stage of the driver circuit consumes a large portion of the power consumption of the driver circuit. The amount of the power consumption varies depending on waveforms of pulse signals provided to the output elements as well as operating speeds of the output elements. Because of the changes in the power consumption, junction temperature of the output elements also changes, which fluctuates the performance of the driver circuit. Consequently, the output amplitudes and edge timings vary from what originally intended.
FIG. 6 shows an example of drain current curves versus gate voltages in the output element formed with CMOS transfer gates when the temperature changes. In general, when the temperature in the MOS transistor devices increases, a threshold voltage denoted by Vt and drain current denoted by Id will decrease. As a result, the drain current Id at the bias point 9 of FIG. 6 decreases. Because of this characteristics, a problem arises in the conventional driver circuit that an output voltage level drops with an elapse of time as shown in FIG. 7A.
Such an output voltage change caused by the temperature change in the output element increases with the increase in the amount of output current flowing to the load. This is because an output impedance of the output element varies with the increase of the temperature, and the output voltage is a product of the output current and the output impedance.
FIGS. 7B-7D show examples of timing deviation between an input signal and an output signal. FIG. 7C shows an intended delay timing of the output signal with respect to the input signal of FIG. 7B. FIG. 7C shows an additional delay time .DELTA.t occurred in the output signal with respect to the input signal of FIG. 7B because of the temperature rise in the output element. In this manner, the timing deviation in the output signal occurs when the temperature in the driver circuit changes. Accordingly, the present invention is directed to a driver circuit which is able to maintain the output impedance of the driver circuit constant as well as to maintain a signal propagation delay time constant.
For a driver circuit that requires a high degree of precision, an external apparatus must be installed to keep the temperature of an area surrounding the driver circuit in a constant value. Examples of such external apparats is a cooler or an air conditioner, which increases the cost and size of the driver circuit.
As explained in the foregoing, in the driver circuit without a temperature compensation capability, the output amplitudes and timings deviate from what originally intended because of the temperature change. Basically, such a temperature change is caused by the change in the power consumption in the output elements 31 and 32 in the driver circuit. Consequently, the driver circuit in the conventional technology is not able to produce output signals having amplitudes and timings with sufficient precision.